By virtue of excellent properties including weather resistance, electric properties, low compression set, heat resistance and freeze resistance, silicone rubber find wide-spread use in a variety of fields such as electric appliances, automobiles, buildings, medical tools and food. Suitable parts made of silicone rubber include, for example, key pads used as rubber contacts in remote controllers, type writers, word processors, computer terminals, and musical instruments; gaskets in buildings; dampers in audio equipment; automobile parts such as connector seals and spark plug boots; packings in compact disk drives; and bread or cake molds. Since the demand for silicone rubber is increasing in recent years, there is a desire to develop a silicone rubber having better properties.
Patent Documents 1 and 2 describe room temperature-curable silicone rubber compositions having UV absorbers added thereto, for use as building sealant. The compositions are of condensation cure type. A photostabilizer and UV absorber are added to impart durable adhesion. No reference is made to transparency.
Patent Document 3 describes a UV-absorbing silicone coating composition. Since this composition is a coating agent, it must be coated onto a molded article to form a coating having UV-absorbing properties. Since a benzotriazole-containing silane is used as the UV absorber, its weather resistance is poor. Patent Document 4 describes a coating agent comprising a UV absorptive group-containing organopolysiloxane. This coating agent must be coated onto a substrate. Its weather resistance is poor. Patent Document 5 describes a room temperature curable coating agent comprising a triazine base UV absorber.
It is well known that liquid dispersions containing titanium oxide particles are effective as the additive for imparting UV-shielding ability. They are used in a variety of fields including coating compositions, cosmetics and hard coat agents. Patent Documents 6 and 7 describe rutile type titanium dioxide particles, which are added to coating compositions. If such particles are added to heat curable silicone rubber compositions, transparency is lost.
A number of methods for improving the heat resistance of silicone rubber are known in the art, for example, a method of blending an inorganic filler to control oxidative degradation and improve heat resistance. For example, Patent Document 8 proposes addition of iron oxide, Patent Document 9 proposes addition of rare earth oxides and hydroxides, and Patent Document 10 proposes addition of surface-treated carbon black. These inorganic fillers, however, have the problem that they detract from transparency.
Metal oxide particles tend to agglomerate together as the average particle size becomes smaller, and when added to heat curable silicone rubber base compounds, adversely affect transparency. Also, when a sol of metal oxide particles dispersed in an organic solvent, alcohol or water is added to heat curable silicone rubber base compounds, the metal oxide particles agglomerate together to detract from transparency.
Patent Document 11 discloses that cerium oxide dispersed in an organosiloxane oil having a phenyl content of 5 to 73% by weight is blended in a silicone rubber base compound at the sacrifice of transparency.
Patent Document 12 proposes to add a cerium oxide powder prepared by the sol-gel method to a heat curable silicone rubber composition for improving its heat resistance. It is described that the cured silicone rubber loaded with 0.05 wt % of cerium oxide powder is transparent. On evaluation of heat resistance, however, 1.5 wt % of cerium oxide powder is added to a silicone rubber composition, which is not transparent in the cured state. Since cerium oxide particles with a smaller particle size agglomerate together with a lapse of time, they detract from transparency when added to a heat curable silicone rubber composition.